A bipartisan group of senators will unveil a stand-alone renewable electricity standard Tuesday in a last-ditch bid to convince leadership that such a controversial bill could move in the partisan Senate.

The actual legislation is expected to be a version of the RES that came out of Bingaman’s panel last year, possibly with a few minor tweaks. That bill would require utilities to provide 15 percent of their power from renewable sources like wind and solar by 2021.

Clean energy advocates, environmentalists and labor groups have been beating the drum for the Senate to take up a standalone version of an RES ever since Senate Democrats abandoned plans to move a sweeping energy and climate bill this summer.

At the time, an RES was one of a number of items on the Democrats’ energy agenda but Majority Leader Harry Reid ultimately decided that it – like a cap-and-trade proposal – was too controversial to move in the divided Senate.

But over recess, Reid suggested that he would be open to revisiting the issue of an RES, even in a lame duck session, which could potentially be a boon to renewable projects in his home state of Nevada. Such statements buoyed the spirits of RES backers, who carried on an aggressive lobbying blitz over the summer.

Brownback was the only Republican senator to expressly offer his support for a renewable standard, although he did so with the caveat that it would have to move alone and that the bill would need to be the one that came out of the energy panel – and not a higher standard advocated by a number of liberal lawmakers.

“I’m not going to go along with just anything [Reid] throws in there,” Brownback told POLITICO last week. “I believe in it. I think it’s the right thing to do, but I can’t be cute with it.”

Sen. Byron Dorgan (D-N.D.) and Brownback teamed up on the Senate floor at the end of July to call for a renewable standard – but at the time the two lawmakers appeared to differ on the strength of it. Dorgan and other Democrats wanted the “strongest possible” standard and called for 20 percent of the nation’s electricity to come from sources like wind, solar and geothermal by 2020.

But RES backers appeared to provide liberal Democrats cover last week when environmentalists, labor and clean energy advocates released an “action statement” backing the weaker of the two proposals.

“The RES passed in 2009 by the Senate Energy and Natural Resources Committee isn’t perfect, but it is the right RES to pass as a starting point at this moment of acute urgency,” the groups said in a letter to Senate leaders.